With recent developments of industry in semiconductors and optoelectronics, use of highly toxic hydrides such as arsine, phosphine, diborane, and hydrogen selenide has markedly increased.
These toxic components are essentially required as raw materials or doping gases in the production of silicon semi-conductors, compound semi-conductors or optical fibers.
Exhaust gases discharged from the process for the production of semi-conductors or optical fibers generally contain unreacted toxic components and, in view of their high toxicity to living organisms, these toxic components must be removed from the exhaust gases before these gases are discharged to open atmosphere in order to prevent environmental pollution. For example, from considerations for working hygiene, threshold limit values (TLV) of arsine, phosphine, diborane, and hydrogen selenide are 0.05 ppm, 0.3 ppm, 0.1 ppm, and 0.05 ppm, respectively.
Known methods for removing such toxic components from the exhaust gas include a wet process which comprises absorbing and decomposing the toxic components in a scrubber, and a dry process which comprises passing the exhaust gas through a column packed with a cleaning agent such as adsorbents or oxidizing agents.
The conventional wet process generally has problems such as corrosion of the apparatus due to the use of an absorbing liquid and difficulty of post-treatment of the absorbing liquid, and thus the wet process is expensive for maintenance of the apparatus.
On the other hand, the conventional process using a cleaning agent comprises oxidative removal of phosphine or arsine by the use of an adsorbing agent, for example, nitrates, e.g., silver nitrate, supported on a porous carrier, or metal chlorides, e.g., ferric chloride, impregnated in a porous carrier, as disclosed in Japanese Patent Application (OPI) No. 89837/81 (the term "OPI" as used herein means a published unexamined patent application). Although this process has solved the problems associated with the above-described wet process, it still has problems such that the exhaust gas discharged from the chemical vapor deposition process (CVD) must be preliminarily subjected to wetting treatment thereby requiring a complicated apparatus.
A further conventional process comprises treating arsine or phosphine with three different absorbents, i.e., inorganic silicates impregnated separately with (1) an aqueous solution of an alkali, (2) an aqueous solution of an oxidizing agent, and (3) an aqueous solution of an alkali and an oxidizing agent, as disclosed in Japanese Patent Publication No. 49822/84. However, this process also involves treatments under wet conditions and, therefore, has the same disadvantages as described above for the wet process.
It has also been proposed to remove arsenic compounds from hydrocarbons mainly comprising propylene in a dry process, in which the hydrocarbons are contacted with a molded catalyst consisting of copper oxide, manganese dioxide or a mixture thereof as disclosed in Japanese Patent Application (OPI) No. 77627/82. However, the molded catalyst obtained directly from copper oxide or manganese dioxide is so brittle that it is broken during use, causing incorporation of the powderous fragments into gas or clogging of the adsorbing column. A great increase of a molding density may somewhat eliminate these disadvantages but, in turn, seriously reduces gas cleaning capacity of the catalyst.
In the chemical war, arsine had been removed by a dry process using a gas mask filled with activated carbon. By taking advantage of adsorption ability of activated carbon, various attempts have been made to improve the performance of activated carbon by incorporating various materials into activated carbon by absorption. For example, U.S. Pat. No. 4,532,115 discloses adsorbing agents for removing arsine or phosphine which comprises activated carbon as a carrier having incorporated therein a copper compound, an alkali metal compound, an alkaline earth metal compound, and at least one compound of Al, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Cd and Pb. Although these adsorbing agents are advantageous in that they can be used in a completely dry state and that they are strong owing to the use of activated carbon as a carrier, their capability of removing arsine, etc. is relatively low. Moreover, since these adsorbing agents generate heat upon contact with air after adsorption of arsine, etc., involving a risk that the activated carbon may ignite depending on conditions, they are limited in conditions for industrial use.
In order to eliminate the disadvantages associated with the above-described conventional techniques, the inventors previously proposed a method for cleaning an exhaust gas comprising contacting the exhaust gas with a molded cleaning agent composed of (1) cupric oxide and (2) at least one metal oxide selected from the group consisting of silicon oxide, aluminum oxide and zinc oxide, as disclosed in EPC 0194366 (corresponding to Japanese Patent Application (OPI) No. 209030/86 and U.S. Pat. No. 4,743,435).
The above-mentioned cleaning agent has an advantage of a markedly higher cleaning capacity, i.e., a maximum volume of a toxic component which can be removed per unit weight or unit volume (hereinafter referred to as saturated cleaning capacity), as compared with the conventional cleaning agents.
However, in spite of the high saturated cleaning capacity, this cleaning agent (hereinafter called a high capacity type cleaning agent) has been proved incapable of achieving a sufficient rate of cleaning depending on conditions, for example, when used at low temperatures such as 10.degree. C. or lower. In other words, in cases where a cleaning column packed with this cleaning agent is used outdoor in winter, breakthrough is reached in a relatively short period of time when used at a low temperature under a high load, resulting in leakage of toxic components in low concentrations at the outlet of the column.